Enhanced microprocessor interconnect with bit shadowing

ABSTRACT

Shadow selection logic is used to select a driver bit position as a shadowed driver value, and line drivers to transmit data for the selected driver bit position and the shadowed driver value on separate link segments of a bus. In addition shadow compare logic is used to compare a selected received value with a shadowed received value from the bus and identify a miscompare in response to a mismatch of the compare, and shadow counters to count a rate of the miscompare relative to a bus error rate over a period of time. A defective link segment is identified in response to the rate of the miscompare within a predefined threshold of the bus error rate.

BACKGROUND

This invention relates generally to computer system communications, and more particularly to providing an enhanced microprocessor interconnect with bit shadowing.

Overall computer system performance is affected by each of the key elements of the computer structure, including the performance/structure of the processor(s), any memory cache(s), the input/output (I/O) subsystem(s), the efficiency of the memory control function(s), the main memory device(s), and the type and structure of the interconnect interface(s).

Extensive research and development efforts are invested by the industry, on an ongoing basis, to create improved and/or innovative solutions to maximizing overall computer system performance and density by improving the system/subsystem design and/or structure. High-availability systems present further challenges as related to overall system reliability due to customer expectations that new computer systems will markedly surpass existing systems in regard to mean-time-between-failure (MTBF), in addition to offering additional functions, increased performance, increased storage, lower operating costs, etc. Other frequent customer requirements further exacerbate the computer system design challenges, and include such items as ease of upgrade and reduced system environmental impact (such as space, power and cooling).

SUMMARY

An exemplary embodiment is a processing device that includes shadow selection logic to select a driver bit position as a shadowed driver value, and line drivers to transmit data for the selected driver bit position and the shadowed driver value on separate link segments of a bus. The processing device also includes shadow compare logic to compare a selected received value with a shadowed received value from the bus and identify a miscompare in response to a mismatch of the compare, and shadow counters to count a rate of the miscompare relative to a bus error rate over a period of time. A defective link segment is identified in response to the rate of the miscompare within a predefined threshold of the bus error rate.

Another exemplary embodiment is a processing system including a first processing device in communication with a second processing device via a bus. The first processing device includes shadow selection logic to select a driver bit position as a shadowed driver value, and line drivers to transmit data for the selected driver bit position and the shadowed driver value on separate link segments of the bus. The second processing device includes shadow compare logic to compare a selected received value with a shadowed received value from the bus and identify a miscompare in response to a mismatch of the compare, and shadow counters to count a rate of the miscompare relative to a bus error rate over a period of time. A defective link segment is identified in response to the rate of the miscompare within a predefined threshold of the bus error rate.

An additional exemplary embodiment is a method for providing bit shadowing in a microprocessor interconnect. The method includes selecting a driver bit position as a shadowed driver value, and transmitting data for the selected driver bit position and the shadowed driver value on separate link segments of a microprocessor interconnect bus. The method further includes comparing a selected received value with a shadowed received value from the microprocessor interconnect bus, identifying a miscompare in response to a mismatch of the compare, counting a rate of the miscompare relative to a bus error rate over a period of time, and identifying a defective link segment in response to the rate of the miscompare being within a predefined threshold of the bus error rate.

A further exemplary embodiment is a design structure tangibly embodied in a machine-readable medium for designing, manufacturing, or testing an integrated circuit. The design structure includes shadow selection logic to select a driver bit position as a shadowed driver value, and line drivers to transmit data for the selected driver bit position and the shadowed driver value on separate link segments of a microprocessor interconnect bus. The design structure also includes shadow compare logic to compare a selected received value with a shadowed received value from the microprocessor interconnect bus and identify a miscompare in response to a mismatch of the compare. The design structure further includes shadow counters to count a rate of the miscompare relative to a bus error rate over a period of time, where a defective link segment is identified in response to the rate of the miscompare within a predefined threshold of the bus error rate.

Other systems, methods, apparatuses, design structures and/or computer program products according to embodiments will be or become apparent to one with skill in the art upon review of the following drawings and detailed description. It is intended that all such additional systems, methods, apparatuses, design structures and/or computer program products be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Referring now to the drawings wherein like elements are numbered alike in the several FIGURES:

FIG. 1 depicts a multi-chip module (MCM) with multiple microprocessor cores communicating via busses with bit shadowing that may be implemented by exemplary embodiments;

FIG. 2 depicts a processing system with multiple processing devices in communication via busses with bit shadowing that may be implemented by exemplary embodiments;

FIG. 3 depicts an example of drive-side and receive-side switching logic that may be implemented by exemplary embodiments;

FIG. 4 depicts data assignments for bus lanes that may be implemented by exemplary embodiments;

FIG. 5 depicts data and clock repair logic with bit shadowing that may be implemented by exemplary embodiments;

FIG. 6 depicts bit shadowing logic that may be implemented by exemplary embodiments;

FIG. 7 depicts an example of bit shadowing that may be implemented by exemplary embodiments;

FIG. 8 depicts examples of multiple processing systems in communication via busses with bit shadowing that may be implemented by exemplary embodiments;

FIG. 9 depicts an exemplary process for an enhanced microprocessor interconnect with bit shadowing that may be implemented by exemplary embodiments; and

FIG. 10 is a flow diagram of a design process used in semiconductor design, manufacture, and/or test.

DETAILED DESCRIPTION

As large computer systems become increasing more complex, the number of interconnections between integrated circuits or chips in the computer systems also increases. The number of interconnections, for example between a microprocessor to memory, the microprocessor to other microprocessors, the microprocessors to cache or I/O chips, are increasing to the point where today's large computer systems may have tens of thousands of interconnections between chips within the system. These signals can be carried on metal wires from a transmit chip through a chip carrier, to cards and/or boards, possibly through several connectors to a receiver chip on a second carrier. All of the interconnections and signals should be manufactured and remain defect free over the life of the product; otherwise, a system failure can occur. A single failure or latent defect typically impacts part or all of the computer system's operation.

In an exemplary embodiment, an enhanced microprocessor interconnect with bit shadowing in a computer system is provided. Bit shadowing supports detection of interconnection failures and dynamically reconfigure signals around defects to maintain operation by sending the same signal on multiple physical connections. Furthermore, bit shadowing detects intermittent problems that occur over a period of time. This may apply to any chip-to-chip interface (i.e., a bus) within a computer or a computer system that sends data, control, or address information to communicate, such as a microprocessor interconnect bus.

In an exemplary embodiment, microprocessor interconnect busses can detect intermittent failures occurring over a period of time and repair themselves with failures in data or clock segments of the bus. This allows multiple defects to exist while enabling the system to maintain normal operation. Bit shadowing takes advantage of an otherwise unused spare link segment or bus lane, sending a redundant value for a selected bit position as a shadowed value on a separate link relative to the selected bit position. A comparison between the received versions of the selected bit position and the shadowed value can be used to identify an error in one of the two link segments for the two signals. Since some number of errors can be expected on any communication interface, this may not itself be sufficient to identify a particular link segment as defective. However, a communication channel such as the microprocessor interconnect bus can be characterized with an expected bit error rate per link segment (or bit lane) and an expected bus error rate. When the bus error rate increases beyond an error rate threshold that is considered to be acceptable, bit shadowing can be performed. For a random distribution, each link segment is expected to contribute an approximately equal fractional value to the total bus error rate. Using bit shadowing, one link segment (or bit lane) may be assumed healthy as the selected bit position while its corresponding shadowed value serves as a lane or link segment under test (LUT). If the rate of miscompares between the received versions of the selected bit position and the shadowed value are within a predefined threshold of the bus error rate, then the LUT is identified as defective. If the rate of miscompares matches an expected fractional value of the normal bus error rate, then the LUT is considered healthy. Thus, by rotating through each bit position, the bus can be fully tested to isolate defects. If all link segments (or bit lanes) have miscompare rates below the predefined threshold of the bus error rate, then the clock link segment of the bus can be identified as defective. Once a defective link segment (or bit lane) is identified, sparing logic can be used to switch out the defect and continue with normal operation. Applying bit shadowing to a processing system can improve the reliability of the processing system as failures in different segments between processing devices are accommodated, including data link segments and clock link segments.

Turning now to FIG. 1, an example of a multi-chip module (MCM) 100 that includes multiple microprocessor cores 104 using bit shadowing is depicted. In an exemplary embodiment, the MCM 100 is a single package, such as ceramic module. The microprocessor cores 104 can be separate chips aggregated together to form the MCM 100 or functional modules integrated in the MCM 100. Each of the microprocessor cores 104 includes processing circuitry to read and execute instructions to perform logic functions. The MCM 100 may be incorporated in a host processing system and interfaced to other devices and subsystems as part of a larger processing system. The microprocessor cores 104 can communicate with each other via microprocessor interconnect busses 106. To maximize communication bandwidth and increase reliability and availability of inter-processor communication, there can be redundant microprocessor interconnect busses 106 interconnecting the microprocessor cores 104. In an exemplary embodiment, each microprocessor core 104 is coupled to the other microprocessor cores 104 in the MCM 100 with at least a pair of microprocessor interconnect busses 106.

The microprocessor cores 104 include drive-side switching logic (DSL) 112 and receive-side switching logic (RSL) 114 to control the assignment of specific signals to segments of the microprocessor interconnect busses 106. Each microprocessor interconnect bus 106 may includes connections, also referred to as link segments or bit lanes. The microprocessor interconnect busses 106 may include single-ended segments, differential-ended segments, or a combination thereof. The segments/lanes can be assigned to dedicated signal types, such as data, commands, addresses, and clocks, or segments/lanes can have mixed uses. The segments/lanes may be bi-directional or uni-directional. In an exemplary embodiment, the microprocessor interconnect busses 106 each include spare link segments that can be used to dynamically repair a failed link segment by switching signal assignments such that one or more of the spare link segments become active, as well as to support bit shadowing. Link segment error registers (LSERs) 120 in the microprocessor cores 104 can be used to identify link segment errors and make spare lane selections in the DSL 112 and RSL 114. The DSL 112 can drive signals, while the RSL 114 receives signals. The DSL 112 includes shadow selection logic to select bit positions for shadowing. The RSL 114 includes error detection logic to detect bus errors and miscompares between selected received values and shadowed received values, which can detect and isolate link segments or bit lanes with intermittent defects. Each microprocessor core 104 may include multiple DSL 112/RSL 114 pair for driving and receiving communications on the microprocessor interconnect busses 106, as well as other busses, such as external busses 108.

Although only a single MCM 100 is shown in FIG. 1 with four interconnected microprocessor cores 104, the scope of the invention is not so limited, as there may be any number of microprocessor cores 104 interconnected via microprocessor interconnect busses 106 within the MCM 100 and multiple MCMs interconnected via the external busses 108. Thus, the external busses 108 may be functionally equivalent to the microprocessor interconnect busses 106, but may vary in the number of segments/bit lanes. The external busses 108 can communicate with a variety of interfaces, such as memory, I/O, or other microprocessor cores 104 that can be integrated in other MCMs 100 or in single processing core devices. Moreover, any number of segments/bit lanes can be included in each microprocessor interconnect bus 106, with a different number segments/bit lanes driven in each direction. For example, for a given DSL 112/RSL 114 pair in one of the interconnected microprocessor cores 104, the microprocessor interconnect bus 106 may include 13 bit lanes, 2 spare lanes and a clock lane driven from the DSL 112, but 20 bit lanes, 2 spare lanes and a clock lane received at the RSL 114, with a corresponding interconnected microprocessor core 104 having the opposite configuration interconnected via the microprocessor interconnect bus 106.

In an exemplary embodiment, initially, all lanes are powered-on, tested and aligned during initialization, but defective lanes and unneeded (unused) spares are deactivated during normal run-time operation. System control software, such as firmware can be used to determine transitions between testing bit lanes and selecting bit lanes for repair, which may be performed as part of a system initialization and configuration process prior to commencing normal operation.

When a hard failure in any link segment is detected, a spare link segment is activated and replaces the defective link segment in the microprocessor interconnect bus 106. The system communication format, error detection and protocols may be the same before and after spare lane invocation. When bit shadowing is invoked, intermittent failures can also be detected during normal operation without reverting to initialization. Initialization is typically a rapid sequence of operations over a period of milliseconds, so intermittent errors may not appear during this short test window. Additionally, certain errors may only occur during normal operation when the MCM 100 is subjected to a more dynamic environment of switching high-speed signals. Each segment of the microprocessor interconnect busses 106 can independently deploy their dedicated spares on a per link basis. This maximizes the ability to survive multiple failures in different link segments. The drive-side switching and receive-side switching are performed independently in each direction on the microprocessor interconnect busses 106. Should multiple link segment failures occur such that all spare link segments are required, then bit shadowing may be disabled. For systems that include additional spare lanes, e.g., 3 or more, multiple link segment failures can be handled and bit shadowing kept active provided that at least one spare is available for use in bit shadowing.

In an exemplary embodiment, the spare lanes are tested and aligned during initialization and deactivated during normal run-time operation. Sparing (switching out a failed link segment) can be performed during initialization based on previous lane failure data. Spare link segments can also be selected dynamically by hardware during run-time as part of error recovery. The error recovery may include re-initialization and repair of links by switching out a failed link. System control software can load the LSERs 120 to control selection of signals for each link segment.

A failed link can be detected using a variety of techniques. For example, during initialization one or more patterns can be sent on the microprocessor interconnect busses 106 to verify that received patterns match the transmitted patterns for detecting hard failures. During normal operation, an error correcting code (ECC) or other error detection and/or correction technique can be used to detect a failure. Once a failed link is detected, the MCM 100 may initiate one or more retry operations to confirm the failure, and then attempt to isolate the specific failed link segment. Repair and re-initialization operations can be performed if any retry operation fails. A failed microprocessor core 104 may also indirectly initiate repair and re-initialization in response to detecting a persistent error that is not corrected via a retry. However, if upon a retry the error does not persist, the error may be an intermittent error. When the intermittent error occurs with a sufficiently high frequency, it can be disruptive to normal operation. Moreover, frequent intermittent errors can indicate that a hard failure is imminent. Therefore, bit shadowing can be initiated when a threshold number of retries are detected over a period of time to identify a specific defective link segment or bit lane. Bit shadowing can be particularly helpful if bus errors are detected using techniques that cannot themselves identify a specific bit lane or link segment that is errant (e.g., parity check or a cyclic redundancy check (CRC) vs. ECC).

FIG. 2 depicts a processing system 202 with multiple processing devices 204 in communication via busses with bit shadowing that may be implemented by exemplary embodiments. The processing devices 204 may be embodiments of the MCM 100 with additional bus interfaces. In an alternate embodiment, the processing devices 204 are single core processing devices (e.g., containing one microprocessor core 104 per processing device 204) with multiple bus interfaces capable of executing instructions. The processing system 202 also includes multiple groups of memory modules 206. The memory modules 206 may be dual-inline memory modules (DIMMs) of dynamic random access memory (DRAM), such as double-data rate 3 (DDR3) DRAMs. The processing system 202 can also include multiple interfaces to other processing systems 202, such as system interfaces A 208 and B 210. The processing system 202 may further interface to input/output (I/O) devices via I/O interface 212. Additionally, the processing system 202 can include multiple voltage regulators 214. The voltage regulators 214 may be distributed across the processing system 202 to minimize noise and parasitic effects that can accompany longer trace lengths. Distributing the voltage regulators 214 can also increase reliability, as a single voltage regulator 214 failure does not impact all of the devices in the processing system 202.

In an exemplary embodiment, each processing device 204 includes multiple bus interfaces. Bus interfaces A 216 and B 218 can drive commands and data on busses 220 and 222 to system interfaces A 208 and B 210 respectively. The bus interfaces A 216 and B 218 can both include DSL 112/RSL 114 pairs as described in FIG. 1. The processing devices 204 may also include memory control bus interfaces MC0 224 and MC1 226 to communicate with the memory modules 206 via memory busses 228 and 230 respectively. The memory control bus interfaces MC0 224 and MC1 226 can both include DSL 112/RSL 114 pairs as described in FIG. 1. The processing devices 204 can also include I/O bus interfaces GX0 232 and GX1 234 to communicate with I/O card and planar interfaces of the I/O interface 212 via busses 236, 238, 240, and 242. The I/O bus interfaces GX0 232 and GX1 234 may both include DSL 112/RSL 114 pairs as described in FIG. 1. There can also be multiple cross-processor communication interfaces to communicate between the processing devices 204 of the processing system 202. Processor interconnect interfaces X 244, Y 246, and Z 248 may be used to communicate between the processing devices 204 via busses 250. In an exemplary embodiment, the processor interconnect interfaces X 244, Y 246, and Z 248 include DSL 112/RSL 114 pairs as described in FIG. 1 for bit shadowing. Each of the busses 220, 222, 228, 230, 240, 242, and 250 may be functionally equivalent to the microprocessor interconnect bus 106 of FIG. 1 but vary in width, data rate, and physical characteristics. Thus, a variety of bus interfaces in the processing system 202 may support bit shadowing with DSL 112/RSL 114 pairs at both ends of the respective busses.

Referring to FIG. 3, greater detail of the DSL 112 and the RSL 114 of FIG. 1 is depicted. For purposes of explanation the DSL 112 and the RSL 114 of FIG. 3 are assumed to be in communication from the DSL 112 to the RSL 114, e.g., DSL 112 of one microprocessor core 104 coupled to the RSL 114 of another microprocessor core 104 via link segments of bus 106. In an exemplary embodiment, the DSL 112 includes multiple 3-to-1 driver multiplexers (muxes) 302, and the RSL 114 includes multiple 3-to-1 receiver muxes 304. The driver muxes 302 control switching of specific bits of driver data 306 to driver bus data 308, which is output on bus 106. Similarly, the receiver muxes 304 control switching of specific receiver bus data 310 received via the bus 106 and output the results as received data 312. In the example depicted in FIG. 3, 13 bits of driver data 306 are routed in groups of three to 15 driver muxes 302. The output of the 15 driver muxes 302 includes 2 spare signals that may be a redundant version of one or two bits of the driver data 306. The receiver bus data 310 includes 15 bits that correspond to the driver bus data 308 when no errors are present. The receiver muxes 304 select 13 of the 15 bits of the receiver bus data 310 to output as 13 bits of received data 312. The LSERs 120 interfaced to the DSL 112 and the RSL 114 control selection of specific bits at each of the driver muxes 302 and the receiver muxes 304 respectively.

Initially, in the absence of any defects, the control signals to the driver muxes 302 and the receiver muxes 304 are set to all zero, selecting the 0 inputs. Referring to FIG. 4, the first row 402 indicates bit selections for normal operation. In this example, spare segments sp1 404 and sp0 406 are powered down (unused) to save power. Subsequent rows depict examples of bit pairs, where one link segment is considered to be bad, resulting in a shift to steer the data from the defective link segment to a functional link segment. A similar mux control vector on the receive side selects all zero inputs to the receiver muxes 304. For example, if bit 12 is determined to be bad then the mux controls in the LSERs 120 are set to zeros on the driver muxes 302 for muxes 0 through 11 and mux 13 is set to a 1. This action steers lane 12 data down link segment 13 of the bus 106. On the receive side, muxes 0-11 of the receiver muxes 304 remain unchanged (set to zero) and bit 12 (mux 12 of the receiver muxes 304) is set to a 1 to steer link segment 13 on the bus 106 to bit 12 of the received data 312. The shift in position is depicted in bit selection pair 408.

Similar to the case with bit 12, if bit (link segment 11 of the lane) 11 is determined to be defective, then the mux controls are set to zeros on the driver muxes 302 for muxes 0-10 and mux 12 and mux 13 are set to a 1. This action steers link segment 11 data down link segment 12 and steers link segment 12 data down link segment 13 of the bus 106. On the receive side, muxes 0-10 of the receiver muxes 304 remain unchanged (set to zero) and bit 11 (mux 11 of the receiver muxes 304) is set to a 1 to steer link segment 12 on the bus 106 to bit 11 and similarly mux 12 of the receiver muxes 304 is set to a 1 to steer bit 13 to link segment 12. The shift in position is depicted in bit selection pair 410. This process can be performed using any bit pairs, e.g., bit pairs 412, 414, down to bit pair 416.

In a similar way any bad or defective link segment can be steered to the adjacent link segment. If both link segments 11 and 12 are defective, then both spares sp1 404 and sp0 406 are employed. In this case, mux 14 is set to a logical 2, mux 13 to a logical 1, and muxes 0-10 are kept at their normal logic zero on the driver muxes 302. This action steers bit 12 down link segment 14 and bit 11 down link segment 13. On the receiver muxes 304, mux 12 is set to a logical 2, mux 11 is set to a logical 1, and all other muxes are set to a logical 0. This action steers bit 11 and bit 12 from link segments 13 and 14 back to their correct bit positions. Hence, two defects can be corrected.

In an exemplary embodiment, the mux controls (LSERs 120) are typically not changed during normal operation since altering the signal paths in real time requires precise timing and coordination between both ends of the link when operating at high speed. A bad link segment may be identified prior to functional operation, during an initialization or power on procedure where specific patterns are transmitted down each lane and checked on the received side for proper operation. If during the initialization process a defective link segment is identified, it can be spared out using one of the available spares. A bad link segment may also be detected during functional operation. Using an ECC, for example, the bad link segment can be spared out in a relatively short period of time, avoiding full re-initialization, to allow the defective signal (or signals) to be re-routed and spare bits utilized. The bus 106 can then be brought back online, and functional operation returns. In an alternate exemplary embodiment, link errors are detected in functional operation with no isolation mechanism present, and hence the link is prompted to reinitialize. It is during the reinitialization process, where predetermined patterns are sent down each link segment and the link segments are interrogated as good or bad, that the defective link segment may be discovered and repaired. When the defect is intermittent, occurring only temporarily, then bit shadowing can be used to identify defects in specific link segments or bit lanes, including a clock.

Although FIGS. 3 and 4 have been described in reference to a particular number of segments/lanes in reference to the bus 106 of FIG. 1, the scope of the invention is not so limited. The number of connections within a grouping can be larger or smaller and/or may depend on the physical characteristics of the device (e.g., MCM 100 and/or processing device 204 of FIG. 2) or the bus 106 (or busses of FIG. 2, such as busses 220, 222, 228, 230, 236-242, and/or 250). Further each bus can be made up of one large group or multiple smaller groups.

Referring to FIG. 5, if any of the link segments are assigned as a bus clock then the bus clock can also be repaired. FIG. 5 depicts DSL 502 in communication with RSL 504 via link segments 506. The DSL 502 drives data bits, such as bit n 508, bit n+1 510, bit n+2 512, bit n+3 514, etc., through driver muxes 516, driver latches 518, and line drivers 520 to the link segments 506. Each of the driver muxes 516 can select from multiple signals to drive, as one of the data bits 508-514, a clock 522, or one of the spares 524 and 526. The selection of signals controlling the driver muxes 516 is driven by configuration signals of the LSERs 528 (e.g., config bit n 530, config clk 532, config bit n+1 534, config bit a 536, config bit b 538, config spare 540 and config spare 542). Additionally, shadow selection logic 580 can also be used to control the driver muxes 516 when bit shadowing is enabled. For example, the shadow selection logic 580 may modify values in the LSERs 528 to transmit both a selected driver bit position and a copy of the driver bit position as a shadowed driver value on parallel link segments 506.

On the receive side at RSL 504, the link segments 506 are coupled to receiver circuits 544, which may amplify and otherwise condition the signals received. To support clock sparing, link segments 506 that can carry a clock signal may be driven into receiver muxes 546 without latching and prior to clock distribution 548. This avoids duplication of the clock distribution 548, which distributes the received clock, for example in the MCM 100 or in processing devices 204. The muxing for data repair(s) can also be performed right after the receiver circuits 544, before sampling or after receiver latches 562 as well. Detection of a defective clock can be performed using similar approaches as for bad data link segments. For example, during initialization the clock can be tested looking across multiple data lanes for global functionality or be redriven and sent to another chip (e.g., a processing device 204) with a known good clock to be tested. During normal operation multiple bad bits across the bus may indicate a clock problem and prompt a clock repair. As part of the initialization, the clock 522 may be swapped out or tested. A defective clock may be shifted directly to a spare link segment or can be shifted to an adjacent link segment with subsequent data link segments shifted to utilize one of the spare link segments. If the clock 522 has intermittent failures, then bit shadowing can be used to identify this condition as well. During normal operation with bit shadowing, an intermittent clock failure can be detected as a degradation of the bus bit error rate with all link segments showing approximately the same bit error rate. For data or spare bits, such as bit n 550, bit n+1 552, bit n+2 554, bit n+3 556 and spares 558 and 560, received signals on the receiver circuits 544 are buffered using receiver latches 562 prior to the receiver muxes 546. The selection of signals controlling the receiver muxes 546 is driven by configuration signals of LSERs 564 (e.g., config bit a 566, config bit b 568, config spare 570 and config spare 572). While the receiver muxes 546 are depicted as 2 input muxes, they are effectively made into higher order muxes by staging multiple receiver muxes 546.

The RSL 504 also includes error detection logic 582 with shadow compare logic 584. During bit shadowing, the same data pattern or electrical signals are sent down two different physical wires (link segments 506) at the same time. A logical comparison can then be made between the received data to determine if they compare or miscompare using compare blocks 586. The compare blocks 586 can compare values received via the link segments 506, and may be arranged such that each value received on the link segments 506 can be compared with the next higher or lower bit position. For example, assuming that 13 bus lanes are available to transmit data, the compare blocks 586 can be connected between position 12 and 11, as well as 11 and 10 to cover shifts in either direction relative to position 11. The comparison between the two lanes is then compared to a global error detection mechanism or bus error rate. Further details are depicted in FIG. 6, where the shadow compare logic 584 interfaces with miscompare selector and shadow counters 602. And-gate functions 604 can be interposed between the shadow compare logic 584 and the miscompare selector and shadow counters 602 to select particular comparison sets for bit shadowing. The selected pair is input to the miscompare selector and shadow counters 602, as well as a bus error value from bus error detection 606. In an exemplary embodiment, the bus error detection 606 performs a check on all active link segments 506 other than the link segment 506 assigned as the shadow value. The error detection technique may be a CRC.

Various counters in the miscompare selector and shadow counters 602 calculate rates to identify a defective link segment. Transaction counter 608 counts the number of bus transactions over a period of time, providing a basis for rate calculations. Bus error counter 610 counts the number of bus errors detected by bus error detection 606. Miscompare counter 612 counts the number of miscompares of a selected pair for bit shadowing. Thus, the bus error rate can be calculated as the bus error counter 610 divided by the transaction counter 608, and the rate of miscompares can be calculated as the miscompare counter 612 divided by the transaction counter 608. Alternatively, the bus error counter 610 may only accumulate on retry operations after a failure is detected.

FIG. 7 depicts an example of bit shadowing that may be implemented by exemplary embodiments. While the example depicted in FIG. 7 refers to data assignments for 15 bus lanes including two spares, sp1 702 on lane 13 and sp0 704 on lane 14, the example may be applied to any number of lanes or link segments. Data assignments to bus lanes in row 706 represents default assignments for normal operation, with sp1 702 and sp0 704 unused (and depowered). As bit shadowing is performed, data assignments to bus lanes shift with the position assigned for shadowing rotated across the bus. At step X 708, lane 7 is the shadowed value 710 of selected position 712 in lane 6. The same data is sent down lane 7 as is sent down lane 6 in unison. The functional data from lane 6 is still sent as part of normal operation, with the shadowed value 710 on lane 7 provided for testing. To accommodate shadowing on lane 7, the data assignments are shifted one position, and sp1 702 provides additional capacity for bit position 12. Once lane 7 is confirmed healthy, the shadowed value 710 is shifted by one position and becomes shadowed value 716 in step X+1 714 on lane 6. Shadowed value 716 is compared against selected position 718 for a mismatch. At a higher level, bus-level error detection is performed across lanes 0-5 and 7-13, using for example, a CRC. The CRC calculation can be performed on received data and compared to the CRC transmitted. If a bus error is detected in response to a CRC mismatch, then a comparison can be made between the two copies of lane 6 as to whether the shadowed value 716 equals the value of the selected position 718.

Since the example depicted in FIG. 7 includes 13 bits on the bus under normal circumstances and assuming the bit errors are equally distributed across the bus, then lane 6 would account for approx 1/13 of the total bus errors (CRC checks). The shadowed value 716 (LUT) is assumed to have a bit error rate (BER) of 10**−12 (one error in 10**12 bits) as are all the other lanes too. Hence the ratio of lane 6 miscompares to total bus errors would be ˜ 1/13 or 0.077 in normal operation. If, however, lane 6 had a BER of 10**−8 while all the other bits on the bus maintained their error rate at 10**−12, then the lane 6 errors would saturate the bus error rate and the ratio of lane 6 errors to bus errors would approach 1. If lane 6 was a good lane and another lane on the bus had a BER=10**−8, then the bus error counter 610 would be much greater than lane 6 miscompare counter 612 (approximately 10**4). Hence, a bad lane can be isolated from a good lane by comparing the ratio of the shadow lane error rate or miscompare counter 612 to the overall bus error rate or bus error counter 610. Since the lane error comparison is being made using a shadowing bit, it does not affect normal operation. If errors occur in bursts this correctly detects the failures, unlike a retry or recreation of the problem. Further, if after a period of time lane 6 is shown to be good, then it can be used to support shadowing of a different lane to continue in search of a bad lane. When performing bit shadowing, the timing between the transmit side and receive side in not critical. The lane sparing and switching of the muxes, e.g., muxes 516 and 546, can happen independently and at very different times. A simple, slow coordination and control between the two ends of the link is managed for proper sequencing.

At row 720, normal operation continues with bad lane 722 repaired through shifting data positions onto different lanes. The shifting utilizes the capacity provided by sp1 702 on lane 13, with sp0 704 still available for use. At step 724, shadowing continues around bad lane 726, as bit position 12 is tested as shadowed value 728 on lane 14 of selected position 730. Here, the spare lane sp0 704 is tested. The process continues at step 2 732 while avoiding bad lane 734 to test bit lane 13 carrying bit position 12 as shadowed value 736 of selected position 738. The shadowing continues down the bus one bit at a time. At step Y 740, shadowing is performed around bad lane 742 with bit position 5 shadowed on lane 7 as shadowed value 744 to compare against selected position 746 on lane 5. Using 3:1 muxes or multiple staged muxes can provide switching to continue bit shadow testing around a bad lane, such as bad lane 742.

FIG. 8 depicts examples of multiple processing systems 202 in communication via busses with bit shadowing that may be implemented by exemplary embodiments. In FIG. 8, a variety of system configurations are depicted as 2-drawer 802, 3-drawer 804, 4-drawer 806, and 5-drawer 808, where each drawer contains a processing system 202. As multiple processing systems 202 are interconnected, a greater amount of processing bandwidth is achieved. The system interfaces A 208 and B 210 support connectivity between the processing systems 202. Using multiple busses, such as busses 810 and 812 with multiple link segments/bit lanes bundled in flexible cables, enhanced reliability can be achieved for each of the system configurations, 2-drawer 802, 3-drawer 804, 4-drawer 806, and 5-drawer 808. Redundant busses provide enhanced reliability for bus-wide issues, while problems with individual link segments/bit lanes of the busses, such as busses 810 and 812, can be detected via bit shadowing.

FIG. 9 depicts a process 900 for providing bit shadowing in a microprocessor interconnect that may be implemented as described in reference to FIGS. 1-8. For example, the process 900 may be implemented in the microprocessor cores 104 of FIG. 1 and/or the processing devices 204 of FIG. 2. At block 902, the shadow selection logic 580 of FIG. 5 selects a driver bit position as a shadowed driver value. The shadowed driver value is a copy of the value at the driver bit position, such as shadowed value 710 and selected position 712 of FIG. 7. The LSERs 120 can be used to assign signals to specific link segments 506 or lanes via the driver muxes 516.

At block 904, the line drivers 520 transmit data for the selected driver bit position and the shadowed driver value on separate link segments 506 of the microprocessor interconnect bus 106. The microprocessor interconnect bus 106 may include lanes of link segments. The downstream lanes may include 13 downstream bit lanes, 2 spare downstream bit lanes, and a downstream clock, while the upstream lanes can include 20 upstream bit lanes, 2 spare upstream bit lanes and an upstream clock. Other configurations can also be implemented within the scope of the invention. The microprocessor interconnect bus 106 may also represent the other busses interconnected to the processing device 204 of FIG. 2 to other processing devices 204, memory subsystems (e.g., memory modules 206 or cache memory (not depicted)), or I/O interface 212.

At block 906, the shadow compare logic 584 compares a selected received value with a shadowed received value from the microprocessor interconnect bus 106. The compare can be performed using compare blocks 586 to check for equivalency between the values. At block 908, the miscompare selector and shadow counters 602 of FIG. 6 identifies a miscompare in response to a mismatch of the compare of block 906. At block 910, shadow counters in the miscompare selector and shadow counters 602 count a rate of the miscompare relative to a bus error rate over a period of time, for example, using bus error counter 610, transaction counter 608, and miscompare counter 612. At block 912, the miscompare selector and shadow counters 602 or other logic in the error detection logic 582 identifies a defective link segment in response to the rate of the miscompare being within a predefined threshold of the bus error rate. For instance, the predefined threshold can be a ratio comparison, where a link segment or lane is identified as defective when it accounts for 50% or more of the bus error rate. A clock link segment of the microprocessor interconnect bus 106 can be identified as defective in response to the rate of the miscompare for all data link segments of the microprocessor interconnect bus 106 being below the predefined threshold of the bus error rate and the bus error rate being above an error rate threshold. In other words, when there is a sufficient number of bus errors to demand bit shadowing, if none of the bit lanes contribute substantially to the overall bus error rate, then the most likely source of the error is the clock.

The driver muxes 302 and the receiver muxes 304 can be configured to switch out a defective link segment and to rotate bit shadowing through each link segment or lane. Rotating bit positions associated with the selected driver bit position, the shadowed driver value, the selected received value, and the shadowed received value enables testing of multiple link segments of the microprocessor interconnect bus 106. To conserve power, unused link segments can be depowered, including a defective link segment, once identified. Unused link segments can also be used for other functions, such as sending out-of-band communication and/or test signals.

FIG. 10 shows a block diagram of an exemplary design flow 1000 used for example, in semiconductor IC logic design, simulation, test, layout, and manufacture. Design flow 1000 includes processes and mechanisms for processing design structures or devices to generate logically or otherwise functionally equivalent representations of the design structures and/or devices described above and shown in FIGS. 1-9. The design structures processed and/or generated by design flow 1000 may be encoded on machine readable transmission or storage media to include data and/or instructions that when executed or otherwise processed on a data processing system generate a logically, structurally, mechanically, or otherwise functionally equivalent representation of hardware components, circuits, devices, or systems. Design flow 1000 may vary depending on the type of representation being designed. For example, a design flow 1000 for building an application specific IC (ASIC) may differ from a design flow 1000 for designing a standard component or from a design flow 1000 for instantiating the design into a programmable array, for example a programmable gate array (PGA) or a field programmable gate array (FPGA) offered by Altera® Inc. or Xilinx® Inc.

FIG. 10 illustrates multiple such design structures including an input design structure 1020 that is preferably processed by a design process 1010. Design structure 1020 may be a logical simulation design structure generated and processed by design process 1010 to produce a logically equivalent functional representation of a hardware device. Design structure 1020 may also or alternatively comprise data and/or program instructions that when processed by design process 1010, generate a functional representation of the physical structure of a hardware device. Whether representing functional and/or structural design features, design structure 1020 may be generated using electronic computer-aided design (ECAD) such as implemented by a core developer/designer. When encoded on a machine-readable data transmission, gate array, or storage medium, design structure 1020 may be accessed and processed by one or more hardware and/or software modules within design process 1010 to simulate or otherwise functionally represent an electronic component, circuit, electronic or logic module, apparatus, device, or system such as those shown in FIGS. 1-9. As such, design structure 1020 may comprise files or other data structures including human and/or machine-readable source code, compiled structures, and computer-executable code structures that when processed by a design or simulation data processing system, functionally simulate or otherwise represent circuits or other levels of hardware logic design. Such data structures may include hardware-description language (HDL) design entities or other data structures conforming to and/or compatible with lower-level HDL design languages such as Verilog and VHDL, and/or higher level design languages such as C or C++.

Design process 1010 preferably employs and incorporates hardware and/or software modules for synthesizing, translating, or otherwise processing a design/simulation functional equivalent of the components, circuits, devices, or logic structures shown in FIGS. 1-9 to generate a netlist 1080 which may contain design structures such as design structure 1020. Netlist 1080 may comprise, for example, compiled or otherwise processed data structures representing a list of wires, discrete components, logic gates, control circuits, I/O devices, models, etc. that describes the connections to other elements and circuits in an integrated circuit design. Netlist 1080 may be synthesized using an iterative process in which netlist 1080 is resynthesized one or more times depending on design specifications and parameters for the device. As with other design structure types described herein, netlist 1080 may be recorded on a machine-readable data storage medium or programmed into a programmable gate array. The medium may be a non-volatile storage medium such as a magnetic or optical disk drive, a programmable gate array, a compact flash, or other flash memory. Additionally, or in the alternative, the medium may be a system or cache memory, buffer space, or electrically or optically conductive devices and materials on which data packets may be transmitted and intermediately stored via the Internet, or other networking suitable means.

Design process 1010 may include hardware and software modules for processing a variety of input data structure types including netlist 1080. Such data structure types may reside, for example, within library elements 1030 and include a set of commonly used elements, circuits, and devices, including models, layouts, and symbolic representations, for a given manufacturing technology (e.g., different technology nodes, 32 nm, 45 nm, 90 nm, etc.). The data structure types may further include design specifications 1040, characterization data 1050, verification data 1060, design rules 1070, and test data files 1085 which may include input test patterns, output test results, and other testing information. Design process 1010 may further include, for example, standard mechanical design processes such as stress analysis, thermal analysis, mechanical event simulation, process simulation for operations such as casting, molding, and die press forming, etc. One of ordinary skill in the art of mechanical design can appreciate the extent of possible mechanical design tools and applications used in design process 1010 without deviating from the scope and spirit of the invention. Design process 1010 may also include modules for performing standard circuit design processes such as timing analysis, verification, design rule checking, place and route operations, etc.

Design process 1010 employs and incorporates logic and physical design tools such as HDL compilers and simulation model build tools to process design structure 1020 together with some or all of the depicted supporting data structures along with any additional mechanical design or data (if applicable), to generate a second design structure 1090. Design structure 1090 resides on a storage medium or programmable gate array in a data format used for the exchange of data of mechanical devices and structures (e.g. information stored in a IGES, DXF, Parasolid XT, JT, DRG, or any other suitable format for storing or rendering such mechanical design structures). Similar to design structure 1020, design structure 1090 preferably comprises one or more files, data structures, or other computer-encoded data or instructions that reside on transmission or data storage media and that when processed by an ECAD system generate a logically or otherwise functionally equivalent form of one or more of the embodiments of the invention shown in FIGS. 1-9. In one embodiment, design structure 1090 may comprise a compiled, executable HDL simulation model that functionally simulates the devices shown in FIGS. 1-9.

Design structure 1090 may also employ a data format used for the exchange of layout data of integrated circuits and/or symbolic data format (e.g. information stored in a GDSII (GDS2), GL1, OASIS, map files, or any other suitable format for storing such design data structures). Design structure 1090 may comprise information such as, for example, symbolic data, map files, test data files, design content files, manufacturing data, layout parameters, wires, levels of metal, vias, shapes, data for routing through the manufacturing line, and any other data required by a manufacturer or other designer/developer to produce a device or structure as described above and shown in FIGS. 1-9. Design structure 1090 may then proceed to a stage 1095 where, for example, design structure 1090: proceeds to tape-out, is released to manufacturing, is released to a mask house, is sent to another design house, is sent back to the customer, etc.

The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor.

The capabilities of the present invention can be implemented in software, firmware, hardware or some combination thereof.

As will be appreciated by one skilled in the art, the present invention may be embodied as a system, method or computer program product. Accordingly, the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” Furthermore, the present invention may take the form of a computer program product embodied in any tangible medium of expression having computer usable program code embodied in the medium.

Any combination of one or more computer usable or computer readable medium(s) may be utilized. The computer-usable or computer-readable medium may be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. More specific examples (a non-exhaustive list) of the computer-readable medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CDROM), an optical storage device, a transmission media such as those supporting the Internet or an intranet, or a magnetic storage device. Note that the computer-usable or computer-readable medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via, for instance, optical scanning of the paper or other medium, then compiled, interpreted, or otherwise processed in a suitable manner, if necessary, and then stored in a computer memory. In the context of this document, a computer-usable or computer-readable medium may be any medium that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The computer-usable medium may include a propagated data signal with the computer-usable program code embodied therewith, either in baseband or as part of a carrier wave. The computer usable program code may be transmitted using any appropriate medium, including but not limited to wireless, wireline, optical fiber cable, RF, etc.

Computer program code for carrying out operations of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++ or the like and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The program code may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider).

The present invention is described below with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computer-readable medium that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable medium produce an article of manufacture including instruction means which implement the function/act specified in the flowchart and/or block diagram block or blocks.

The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions.

The diagrams depicted herein are just examples. There may be many variations to these diagrams or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order, or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention.

Module support devices (such as buffers, hubs, hub logic chips, registers, PLL's, DLL's, non-volatile memory, etc) may be comprised of multiple separate chips and/or components, may be combined as multiple separate chips onto one or more substrates, may be combined onto a single package and/or or integrated onto a single device—based on technology, power, space, cost and other tradeoffs. In addition, one or more of the various passive devices such as resistors, capacitors may be integrated into the support chip packages and/or into the substrate, board or raw card itself, based on technology, power, space, cost and other tradeoffs. These packages may also include one or more heat sinks or other cooling enhancements, which may be further attached to the immediate carrier or be part of an integrated heat removal structure that contacts more than one support and/or memory devices.

Memory devices, hubs, buffers, registers, clock devices, passives and other support devices and/or components may be attached as part of the memory modules 206 of FIG. 2 via various methods including solder interconnects, conductive adhesives, socket assemblies, pressure contacts and other methods which enable communication between the two or more devices and/or carriers via electrical, optical or alternate communication means.

The one or more modules, cards and/or alternate subsystem assemblies and/or processing devices 204 may be electrically connected to the processing system 202, processor complex, computer system or other system environment via one or more methods such as soldered interconnects, connectors, pressure contacts, conductive adhesives, optical interconnects and other communication and power delivery methods. Inter-connection systems may include mating connectors (e.g. male/female connectors), conductive contacts and/or pins on one carrier mating with a compatible male or female connection means, optical connections, pressure contacts (often in conjunction with a retaining mechanism) and/or one or more of various other communication and power delivery methods. The interconnection(s) may be disposed along one or more edges of the assembly, may include one or more rows of interconnections and/or be located a distance from an edge of the system depending on such application requirements as the connection structure, the number of interconnections required, performance requirements, ease of insertion/removal, reliability, available space/volume, heat transfer/cooling, component size and shape and other related physical, electrical, optical, visual/physical access, etc. Electrical interconnections on contemporary modules are often referred to as contacts, pins, tabs, etc. Electrical interconnections on a contemporary electrical connector are often referred to as contacts, pads, pins, pads, etc.

Information transfers (e.g. packets) along a bus, channel, link or other interconnection means may be completed using one or more of many signaling options. These signaling options may include one or more of such means as single-ended, differential, optical or other communication methods, with electrical signaling further including such methods as voltage and/or current signaling using either single or multi-level approaches. Signals may also be modulated using such methods as time or frequency, non-return to zero, phase shift keying, amplitude modulation and others. Signal voltage levels are expected to continue to decrease, with 1.5V, 1.2V, 1V and lower signal voltages expected, as a means of reducing power, accommodating reduced technology breakdown voltages, etc.—in conjunction with or separate from the power supply voltages. One or more power supply voltages, e.g. for DRAM memory devices, may drop at a slower rate that the I/O voltage(s) due in part to the technological challenges of storing information in the dynamic memory cells.

One or more clocking methods may be utilized within the processing system 202, including global clocking, source-synchronous clocking, encoded clocking or combinations of these and other methods. The clock signaling may be identical to that of the signal (often referred to as the bus “data”) lines themselves, or may utilize one of the listed or alternate methods that is more conducive to the planned clock frequency(ies), and the number of clocks required for various operations within the system/subsystem(s). A single clock may be associated with all communication to and from the memory, as well as all clocked functions within the system, or multiple clocks may be sourced using one or more methods such as those described earlier. When multiple clocks are used, the functions within the system may be associated with a clock that is uniquely sourced to the system and/or may be based on a clock that is derived from the clock included as part of the information being transferred to and from the system (such as that associated with an encoded clock). Alternately, a unique clock may be used for the information transferred to the system, and a separate clock for information sourced from one (or more) of the systems. The clocks themselves may operate at the same or frequency multiple of the communication or functional frequency, and may be edge-aligned, center-aligned or placed in an alternate timing position relative to the data, command or address information.

The use of bus termination, on busses ranging from point-to-point links to complex multi-drop structures, is becoming more common consistent with increased performance demands. A wide variety of termination methods can be identified and/or considered, and include the use of such devices as resistors, capacitors, inductors or any combination thereof, with these devices connected between the signal line and a power supply voltage or ground, a termination voltage (such voltage directly sourced to the device(s) or indirectly sourced to the device(s) from a voltage divider, regulator or other means), or another signal. The termination device(s) may be part of a passive or active termination structure, and may reside in one or more positions along one or more of the signal lines, and/or as part of the transmitter and/or receiving device(s). The terminator may be selected to match the impedance of the transmission line, be selected as an alternate impedance to maximize the useable frequency, signal swings, data widths, reduce reflections and/or otherwise improve operating margins within the desired cost, space, power and other system/subsystem limits.

Technical effects and benefits include providing bit shadowing for interconnections in a computer system. Shadowing can be used to isolate and determine a bad bit lane or link segment over much longer periods of time than simply during an initialization or retry recovery process. This is important when relatively low bit error rates exist that may not be detected during an initialization, which can last for only a few milliseconds. The bit shadowing can run in real time with functional traffic and workloads in order to capture more complex and sophisticated events that cannot be recreated or might not occur with a re-initialization, such as chip noise, switching of other circuits or I/O, etc. Bad lanes or link segments can be switched in and out dynamically with little or no precision timing required between the two ends of the link.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. 

1. A processing device comprising: shadow selection logic to select a driver bit position as a shadowed driver value; line drivers to transmit data for the selected driver bit position and the shadowed driver value on separate link segments of a bus; shadow compare logic to compare a selected received value with a shadowed received value from the bus and identify a miscompare in response to a mismatch of the compare; and shadow counters to count a rate of the miscompare relative to a bus error rate over a period of time, wherein a defective link segment is identified in response to the rate of the miscompare within a predefined threshold of the bus error rate.
 2. The processing device of claim 1 wherein the bus includes multiple data link segments, a clock link segment, a first spare link segment to provide capacity for the shadowed driver value and a second spare link segment selectable to replace one of the data link segments or the clock link segment.
 3. The processing device of claim 1 wherein bit positions associated with the selected driver bit position, the shadowed driver value, the selected received value, and the shadowed received value are rotated to test multiple link segments of the bus.
 4. The processing device of claim 1 wherein the bus interconnects to a second processing device.
 5. The processing device of claim 4 wherein the processing device is a microprocessor core of a multi-chip module (MCM) and the second processing device is a second microprocessor core of the MCM.
 6. The processing device of claim 4 wherein the line drivers of the processing device transmit data to the shadow compare logic of the second processing device, and the line drivers of the second processing device transmit data to the shadow compare logic of the processing device.
 7. The processing device of claim 1 wherein the bus error rate is determined as a ratio of bus errors to bus transactions, and further wherein the bus errors are determined using cyclic redundancy checks (CRCs) of data on the link segments of the bus excluding the link segment upon which the shadowed received value is received.
 8. The processing device of claim 1 wherein an unused link segment is depowered.
 9. A processing system comprising: a first processing device comprising: shadow selection logic to select a driver bit position as a shadowed driver value; and line drivers to transmit data for the selected driver bit position and the shadowed driver value on separate link segments of a bus; and a second processing device in communication with the first processing device via the bus, wherein the second processing device comprises: shadow compare logic to compare a selected received value with a shadowed received value from the bus and identify a miscompare in response to a mismatch of the compare; and shadow counters to count a rate of the miscompare relative to a bus error rate over a period of time, wherein a defective link segment is identified in response to the rate of the miscompare within a predefined threshold of the bus error rate.
 10. The processing system of claim 9 wherein the bus includes multiple data link segments, a clock link segment, a first spare link segment to provide capacity for the shadowed driver value and a second spare link segment selectable to replace one of the data link segments or the clock link segment.
 11. The processing system of claim 10 wherein the clock link segment is identified as defective in response to the rate of the miscompare for all of the data link segments being below the predefined threshold of the bus error rate and the bus error rate being above an error rate threshold.
 12. The processing system of claim 9 wherein bit positions associated with the selected driver bit position, the shadowed driver value, the selected received value, and the shadowed received value are rotated to test multiple link segments of the bus.
 13. The processing system of claim 9 wherein redundant busses interconnect the first processing device with the second processing device.
 14. The processing system of claim 13 wherein the first processing device and the second processing device both include the shadow selection logic, the line drivers, the shadow compare logic, and the shadow counters, and further wherein the line drivers of the first processing device transmit data to the shadow compare logic of the second processing device, and the line drivers of the second processing device transmit data to the shadow compare logic of the first processing device.
 15. The processing system of claim 9 wherein the bus error rate is determined as a ratio of bus errors to bus transactions, and further wherein the bus errors are determined using cyclic redundancy checks (CRCs) of data on the link segments of the bus excluding the link segment upon which the shadowed received value is received.
 16. The processing system of claim 9 wherein the first processing device and the second processing device are microprocessor cores of a multi-chip module (MCM).
 17. A method for providing bit shadowing in a microprocessor interconnect, the method comprising: selecting a driver bit position as a shadowed driver value; transmitting data for the selected driver bit position and the shadowed driver value on separate link segments of a microprocessor interconnect bus; comparing a selected received value with a shadowed received value from the microprocessor interconnect bus; identifying a miscompare in response to a mismatch of the compare; counting a rate of the miscompare relative to a bus error rate over a period of time; and identifying a defective link segment in response to the rate of the miscompare being within a predefined threshold of the bus error rate.
 18. The method of claim 17 further comprising: rotating bit positions associated with the selected driver bit position, the shadowed driver value, the selected received value, and the shadowed received value to test multiple link segments of the microprocessor interconnect bus.
 19. The method of claim 17 wherein the microprocessor interconnect bus connects a microprocessor to one of: another microprocessor, a memory subsystem, or an input/output (I/O) interface.
 20. The method of claim 17 wherein a clock link segment of the microprocessor interconnect bus is identified as defective in response to the rate of the miscompare for all data link segments of the microprocessor interconnect bus being below the predefined threshold of the bus error rate and the bus error rate being above an error rate threshold. 